Method and system for knock control

Abstract

Methods and systems are provided for monitoring knock in a variable displacement engine. During fuel-cut operation, an engine cylinder may be heated. Upon fuel reactivation, a crankshaft sensor profile may be relearned responsive to lack of knock in the heated cylinder. In this way, erroneous identification of a knocking cylinder due to inaccurate crankshaft position data may be determined and mitigated by re-learning the crankshaft sensor profile.

The invention claimed is: 
     
       1. A system, comprising:
 an engine; 
 a crankshaft position sensor configured to measure a position and speed of the engine; 
 a knock sensor configured to measure vibrations of the engine; 
 a variable displacement engine (VDE) mechanism configured to selectively disable one or more cylinders of the engine; and 
 a controller storing instructions in non-transitory memory executable to:
 during fuel-cut operation, heat a first cylinder of the engine by adjusting the VDE mechanism to hold cylinder valves of the first cylinder closed while a piston of the first cylinder compresses trapped gas in the first cylinder; and 
 upon fuel reactivation, re-learning a profile of the crankshaft position sensor responsive to lack of knock in the heated first cylinder, the lack of knock detected based on output from the knock sensor. 
 
 
     
     
       2. The system of  claim 1 , wherein the fuel reactivation includes adjusting the VDE mechanism to actuate the cylinder valves of the heated first cylinder, injecting fuel to the heated first cylinder, and applying spark to the heated first cylinder. 
     
     
       3. The system of  claim 1 , wherein the instructions are executable to notify an operator and/or set a diagnostic code indicating that the heated first cylinder is a persistently-knocking cylinder responsive to detecting knock in the heated first cylinder upon fuel reactivation. 
     
     
       4. The system of  claim 1 , wherein the instructions are executable to identify that the first cylinder is a persistently knocking cylinder during a prior fueled engine operation by:
 determining, based on output from the knock sensor, that a cylinder of the engine is knocking; 
 identifying that the first cylinder of the engine is knocking based on output from the crankshaft position sensor; 
 retarding spark timing of the first cylinder; and 
 determining that the first cylinder is a persistently-knocking cylinder upon output from the knock indicating that the first cylinder is still knocking. 
 
     
     
       5. The system of  claim 1 , wherein the instructions are executable to re-learn the profile of the crankshaft position sensor during an engine-off condition when a vehicle in which the engine is installed is parked on level ground. 
     
     
       6. The system of  claim 5 , wherein to re-learn the profile, the instructions are executable to:
 crank the engine at a constant speed while a transmission of the engine is in neutral; 
 sample output from the crankshaft position sensor; 
 calculate and normalize half-period times for each tooth of the crankshaft position sensor based on the output from the crankshaft position sensor; and 
 update a table stored in memory of the controller with the normalized half-period times. 
 
     
     
       7. A method, comprising:
 during fuel-cut operation, heating an engine cylinder; and 
 upon fuel reactivation, re-learning a crankshaft sensor profile responsive to lack of knock in the heated cylinder. 
 
     
     
       8. The method of  claim 7 , wherein heating the engine cylinder during fuel-cut operation comprises holding cylinder valves of the engine cylinder closed while fuel injection and spark to the engine cylinder are disabled, thereby heating the engine cylinder via compression of trapped cylinder gas by a piston of the engine cylinder. 
     
     
       9. The method of  claim 8 , further comprising upon heating the engine cylinder, fuel reactivating the heated cylinder by actuating the cylinder valves of the heated cylinder, injecting fuel to the heated cylinder, and applying spark to the heated cylinder; and indicating a lack of knock in the heated cylinder responsive to output from a knock sensor being below a knock threshold upon the fuel reactivation. 
     
     
       10. The method of  claim 9 , wherein the crankshaft sensor profile is a profile of a crankshaft position sensor operable to measure engine position, and further comprising identifying that the engine cylinder is knocking during a previous fueled engine operation based on output from the knock sensor and based on output from the crankshaft position sensor. 
     
     
       11. The method of  claim 9 , further comprising notifying an operator and/or setting a diagnostic code indicating that the heated cylinder is a persistently-knocking cylinder responsive to detecting knock in the heated cylinder upon fuel reactivation. 
     
     
       12. The method of  claim 7 , wherein the engine cylinder is a first cylinder, and further comprising responsive to the lack of knock in the heated first cylinder, heating a second cylinder and confirming if the second cylinder knocks upon fuel reactivation to the second cylinder. 
     
     
       13. The method of  claim 7 , wherein the engine cylinder is an engine cylinder of an engine of a vehicle, and wherein the fuel-cut operation comprises operation of the vehicle with operator-requested torque below a threshold where fuel supply to the engine is disabled and the engine still rotates. 
     
     
       14. A method, comprising:
 during fueled engine operation, identifying a persistently-knocking cylinder of an engine; 
 during unfueled engine operation, heating each cylinder of the engine; 
 reactivating each cylinder one-by-one to identify if any cylinders of the engine are induced to knock; and 
 indicating a change in performance of a crankshaft position sensor responsive to determining that the persistently-knocking cylinder is not induced to knock upon reactivation and an adjacent cylinder in an engine firing order is induced to knock upon reactivation. 
 
     
     
       15. The method of  claim 14 , further comprising responsive to determining that the persistently-knocking cylinder is induced to knock upon reactivation, indicating that the persistently-knocking cylinder is degraded, and in response, notifying an operator and/or adjusting one or more engine operating parameters. 
     
     
       16. The method of  claim 14 , wherein identifying the persistently-knocking cylinder during fueled engine operation comprises initiating combustion in one or more cylinders of the engine according to the engine firing order and determining, based on output from a knock sensor being greater than a knock threshold, that a cylinder of the one or more cylinders is knocking. 
     
     
       17. The method of  claim 16 , further comprising:
 identifying which cylinder of the one or more cylinders is knocking based on output from the crankshaft position sensor; 
 retarding spark timing of the identified cylinder; and 
 if output from the knock sensor is still greater than the knock threshold, determining that the identified cylinder is the persistently-knocking cylinder. 
 
     
     
       18. The method of  claim 14 , wherein heating each cylinder of the engine and reactivating each cylinder one-by-one to identify if any cylinders of the engine are induced to knock comprises:
 disabling fuel injection and spark to all cylinders of the engine; 
 holding cylinder valves of each cylinder closed while fuel injection and spark to each cylinder are disabled and heating each cylinder via compression of trapped cylinder gas by a respective piston; and 
 reactivating each cylinder one-by-one by actuating cylinder valves for a reactivated cylinder, injecting fuel to the reactivated cylinder, activating spark in the reactivated cylinder, and repeating the actuating, injecting, and activating successively for each remaining cylinder. 
 
     
     
       19. The method of  claim 14 , further comprising responsive to indicating the change in performance of the crankshaft position sensor, relearning crankshaft position sensor tooth location. 
     
     
       20. The method of  claim 19 , wherein relearning crankshaft position sensor tooth location comprises, during an engine-off condition when a vehicle in which the engine is installed is parked on level ground:
 cranking the engine at a constant speed while a transmission of the engine is in neutral; 
 sampling output from the crankshaft position sensor; 
 calculating a distance between each tooth of the crankshaft position sensor based on the output from the crankshaft position sensor; and 
 updating a table stored in memory of a controller of the vehicle with the calculated distance between each tooth.